The multi-billion dollar global market for semiconductor defect management is growing both in absolute terms and as a percentage of semiconductor capital equipment investment. In general, there are two factors that determine the economics of a semiconductor fabrication facility at a given utilization level, namely throughput and yield. As complex new technologies such as 300 mm wafers, copper interconnects, and reduced feature (circuit) sizes drive the margin of error in fabrication ever lower, new inspection technologies are critical to keep yields high and bottom-line economics attractive. Detection and elimination of chemical contamination and other types of defects is a constant concern for semiconductor manufacturers and equipment suppliers. Contamination can arise from use of processing chemicals, processing equipment and poor handling techniques. Contaminants can include for example metals, carbon and organic compounds. Other types of defects can result from a wide range of causes, including flaws in the semiconductor crystal, improper processing, improper handling, and defective materials. In addition, many cleaning steps are required in semiconductor wafer fabrication. Each step is time consuming and requires expensive chemicals that may require special disposal procedures. Existing methods for monitoring or controlling these processes are expensive and time consuming. As a result, wafers are often cleaned for a longer period of time and using more chemicals than are required.
Defect detection and characterization systems can be divided into in-line and off-line systems. “In-line” refers to inspection and measurement that takes place inside the clean room where wafers are processed. “Off-line” refers to analysis that takes place outside of the wafer processing clean room, often in a laboratory or separate clean room that is located some distance from the manufacturing area. In addition, many of these analytical techniques are destructive, which requires either the sacrifice of a production wafer or the use of expensive “monitor” wafers for analysis. In-line inspection and measurement is crucial for rapidly identifying and correcting problems that may occur periodically in the manufacturing process. A typical wafer can undergo over 500 individual process steps and require weeks to complete. Each wafer can have a finished product value of up to $100,000. Because the number of steps, and period of time, involved in wafer fabrication are so large, a lot of work in process can exist at any point in time. It is critical that process-related defects be found and corrected immediately before a large number (and dollar value) of wafers are affected.
Many types of defects and contamination are not detectable using existing in-line tools, and these are typically detected and analyzed using “off line” techniques (described below) such as Total Reflectance X-ray Fluorescence (TXRF), Vapor Phase Decomposition Inductively Coupled Plasma-Mass Spectrometry (VPD ICP-MS) or Secondary Ion Mass Spectrometry (SIMS). Since these techniques are used off-line (outside of the clean room used to process wafers) and usually occur hours, or even days, after the process step that has caused the contamination, their value is significantly limited.
A brief description of some well known techniques for wafer inspection and chemical contamination detection are presented in Table 1. This list is not in any sense exhaustive as there are a very large number of techniques that are used for some type of semiconductor analysis or characterization.
TABLE 1Analytical In-line/TechniqueDescriptionOff-lineTotal ReflectionX-rays irradiate the wafer within theOff-lineX-Raycritical angle for total external reflectance,Fluorescencecausing surface atoms to fluoresce.(TXRF)AutomatedOptical images are acquired andIn-lineOpticalautomatically analyzed for detection ofMicroscopylarge defects.LaserWafer surface is illuminated with laserIn-lineBackscatteringspots and the angle and/or polarization ofreflected light is analyzed to detect andclassify particles.Vapor PhaseWafers “scanned” with a drop of HF that isOff-lineDecompositionanalyzed using mass spectrometry.InductivelyCoupled-MassSpectrometry(VPD ICP-MS)Secondary IonIon beam sputters the wafer surfaceOff-lineMasscreating secondary ions that are analyzedSpectroscopyin a mass spectrometer.(SIMS)
Table 2 summarizes some major advantages and disadvantages of each technique. In general, off-line detection techniques are extremely sensitive to tiny amounts of contamination; but are slow, expensive and complex to operate. Some have limited, or no, imaging or surface mapping capability, or are destructive in nature. In-line techniques are much faster, non-destructive and provide defect mapping, but have limited chemical contamination detection or analysis capability.
TABLE 2Analytical TechniqueAdvantagesDisadvantagesTotal Reflection X-RayVery sensitiveSlow (>1 hour/wafer)Fluorescence (TXRF)Some mapping capabilityLimited coverageNondestructiveUnpatterned wafers onlyAutomated OpticalFastVery limited chemical andMicroscopyRelatively low costparticle detectionDetects a wide range of macrodefects (>50 microns)Imaging of wafer surfaceNon-contact/non-destructiveLaser BackscatteringFastOnly detects particles - noRelatively low costchemistryDetects very small particlesImaging of water surfaceNon-contact/non-destructiveVapor PhaseVery sensitiveDestructiveDecompositionAble to identifySlowInductively Coupled-wide range of contaminantsExpensiveMass SpectrometryOnly works on bare siliconComplex(VPD ICP-MS)Cannot imageSecondary IonVery sensitiveExpensiveMass Spectroscopy (SIMS)Detects a wide range ofSlowcontaminantsDestructiveDetects sub-surface
In general, existing in-line wafer inspection tools operate at production speeds and generate images of the wafer surface that are processed to identify and locate defects. These techniques, however, are as mentioned above very limited in their ability to detect chemical contamination. Laser backscattering systems are limited to detecting particles down to sub-micron sizes, and optical microscopy systems can only detect chemical contamination that results in a visible stain or residue. Both techniques lack the ability to identify or classify the chemical composition of the particle or contamination. Off-line laboratory techniques are used to qualify the cleanliness of new processes and equipment, or to analyze defects detected by in-line equipment or as part of failure analysis. A critical need therefore exists for a fast, inexpensive and effective means of detecting, locating and classifying relatively small quantities of chemical contamination on production wafers.
It is therefore an object of the invention to provide an improved method and system for inspection of surfaces of materials, such as semiconductor wafers.
It is an additional object of the invention to provide an improved method and system for providing images of surface defects on an semiconductor wafer.
It is yet another object of the invention to provide an improved method and system for identifying different classes of semiconductor wafer surface defects by pattern recognition.
It is still a further object of the invention to provide an improved method and system for classifying categories of surface defects on semiconductor wafers, including without limitation surface defect states, electrostatic field variations, oxide states and chemical contamination.
It is also an additional object of the invention to provide an improved method and system for sensing electrostatic fields arising from semiconductor wafer surface defects.
It is yet another object of the invention to provide an improved method and system for detecting the presence of thin dielectric films on surfaces of semiconductor wafers and to detect film defects such as pinholes, bubbles, delaminations, or contamination under the film.
It is a further object of the invention to provide an improved method and system to sense variations in oxide states on semiconductor wafer surfaces.
It is also a further object of the invention to provide an improved method and system to classify particulate contaminants on semiconductor wafers identified initially by optical inspection systems.
It is yet a further object of the invention to provide an improved method and system for detecting variations in dopant concentration of semiconductor wafers.
It is another object of the invention to provide an improved method and system for use of an NVCPD sensor to inspect the surface quality of semiconductor wafers.
It is still another object of the invention to provide an improved method and system of NVCPD sensors in combination with other inspection systems for evaluating semiconductor wafer surface properties.
It is a further object of the invention to provide an improved method and system for producing topological images of differing contact potential characteristic of defects on a semiconductor wafer.
It is also an object of the invention to provide an improved method and system for rapidly scanning the surface of a semiconductor wafer to identify sub-microscopic, microscopic and macroscopic surface defects characterized by potential field disturbances on the wafer surface.
It is also an object of the invention to provide an improved method and system for detecting the cleanliness of a semiconductor wafer to determine if a cleaning process has eliminated all contaminants and to avoid the time and expense of cleaning wafers for longer than is necessary to remove contaminants.
In each case described above, wafer surface can refer to the front-side (patterned side) of the wafer, back-side (unpatterned side) of the wafer, or the edge of the wafer.
Other objects, features and advantages of the present invention will be readily apparent from the following description of the preferred embodiment thereof, taken in conjunction with the accompanying drawings described below.